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We have developed a new millimeter wave receiver sy stem with input optics that enables simultaneous observations in four bands at 22, 43, 86 and 129 GHz. The functional goal of the optics is to facilitate cali bration of tropospheric phase fluctuations in millimeter-wave VLBI observations. To make the beams of the four bands point the same position in the sky, it is cru cial that errors in the alignment of these beams remain small. On-site test observations showed that the beam centers for the four bands, with reference to the 86 GHz beam c enter, are aligned within 2 arcseconds over most of the elevation range of th e Korean VLBI Network 21-m telescopes. Measured telescope aperture efficiencie s including multiband receiver optics are 65% at 22 GHz, 62% at 43 GHz, 57% at 86 GHz and 38% at 129 GHz.

Korean VLBI Network receiver optics for simultaneous multi-frequency observation